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Neuronal stem cells, originally viewed a source of spare parts that would rebuild damaged circuits, are proving instead to function more like capable paramedics, coming to the rescue of degenerating neurons in multiple animal models of disease. How stem cells aid sick neurons is unclear, but in a paper to be published in PNAS online, Eric Herlenius of Karolinska Institute in Stockholm, Sweden; Richard Sidman and colleagues at Harvard Medical School; and Evan Snyder at the Burnham Institute for Medical Research, La Jolla, California, propose that stem cells initiate aid to neurons by direct cell-to-cell communication via gap junctions. Forming within hours after stem cells contact host neurons, and long before any sign of synaptic connection between the two, the gap junctions serve to modulate network activity, suppress inflammatory damage, and inhibit neuron death, the investigators show. The results suggest that gap junctions comprise an important first step in the assimilation of engrafted stem cells, nurturing neurons while providing a template for later synaptic communication.

Previous work from the same labs suggested that the mechanism by which stem cells salvage neurons requires cell-to-cell contact (see ARF related news story on Li et al., 2006), and similar results have been reported in a primate model of Parkinson disease (Redmond et al., 2007). To get a glimpse at the earliest stages of stem cell-host cell contact, first authors Johan Jäderstad and Linda Jäderstad used an in vitro system where they added green fluorescent protein-labeled mouse neural stem cells to rodent striatal slice cultures. After one month, two-thirds of the stem cells displayed membrane Na+/K+ channel activity indicative of a neuronal phenotype, but none of them showed repetitive action potentials characteristic of mature neurons. Nonetheless, the engrafted cells underwent spontaneous calcium fluctuations in concert with host neurons, suggesting they had integrated into neural networks.

Because there was no evidence of synaptic connections between the new and old cells, the researchers looked for other means of communication. A prime candidate was gap junctions, clusters of connexin proteins that form channels on the plasma membrane, link the cytosols of coupled cells, and allow the exchange of small (The results suggested that gap junctions were responsible for early cell-cell communication and network formation, but was that contact beneficial for the host neurons? The answer appears to be yes. Culturing striatal slices normally results in a level of neuron death and gliosis, but adding stem cells was able to reduce this damage. However, blocking gap junction formation with connexin43 siRNA, or blocking channel function with inhibitors, resulted in a return to normal levels of neuronal damage and death.

The same effect was seen in vivo in two different mouse models of neurodegeneration. In SCA1 mice, loss of the protein ataxin-1 leads to degeneration of cerebellar Purkinje neurons and ataxia, which can be prevented by transplanting neural stem cells. In those mice, both histological rescue of neurons and functional improvements occurred even though none of the stem cells differentiated into mature Purkinje neurons. Instead, the transplanted cells made extensive gap junction connections with host neurons. In the second model, the “nervous” mutant mouse, it was known that transplanting stem cells rescues neurons by reducing pathologically high levels of cerebellar tissue plasminogen activator (tPA). In this case, the investigators found that stem cells that did not express connexin43 were unable to form gap junctions or to rescue neurons.

The results indicate that gap junctions are a critical early step in both the integration of grafted neurons into host circuits and in protection of neurons from death. The authors propose that intracellular networks formed via gap junctions are “an early form of communication that precedes and sets the stage for later electrochemical synapses and ‘traditional’ electrophysiological communication between grafted and host cells, mimicking the connexin-mediated interaction of endogenous progenitors with their neighboring cells during CNS development.” In support of this idea, they show evidence that a similar coupling occurs upon transplantation of neuronal stem cells after spinal cord injury in rats, suggesting that their results with striatal and cerebellar neurons may be more generally applicable.—Pat McCaffrey